1. Field of the Invention
The present invention relates to a drum brake, and more particularly, to a drum brake having a wheel cylinder for expanding a pair of brake shoes.
2. Discussion of the Related Art
A drum brake used in a vehicle generally includes a wheel cylinder, brake shoes expanded by a force derived from the wheel cylinder to generate a braking force, and an anchor portion that supports the brake shoes and receives an anchor reaction force from the brake shoes.
A brake shoe having one end coupled to the wheel cylinder that is located close to an entrance of the brake, and having another end coupled to the anchor portion that is located close to an exit of the brake, is called a leading shoe. A brake shoe having one end coupled to the anchor portion that is located close to the entrance of the brake, and having another end coupled to an input portion that is located close to the exit of the brake, is called a trailing shoe.
Drum brakes widely used at present include a leading trailing drum brake, a two leading drum brake, and a duo servo drum brake.
The leading trailing drum brake has a pair of oppositely disposed brake shoes. A wheel cylinder for expanding the paired brake shoes is disposed between the first ends of the paired brake shoes. An anchor portion is disposed between the second ends of the paired brake shoes. The leading shoe and the trailing shoe are incorporated into the drum brake.
The leading trailing drum brake has good braking stability, and the assembly of a parking brake into the drum brake structure is easy.
The two leading drum brake uses two leading brake shoes. This type of drum brake may further be classified into a drum brake of a single motion type (TP1W) and a drum brake of a dual motion type (TP2W). The TP1W drum brake produces a braking force at high gain in a forward direction of the rotation of the brake drum, but produces a braking force at less gain in a backward direction than in the forward direction. The TP2W drum brake produces a braking force at high gain in both the forward and backward directions.
The two leading drum brake is disadvantageous in that since two wheel cylinders are used, its manufacturing cost is high and the parking brake mechanism is complicated.
The duo servo drum brake has two brake shoes (a primary shoe and a secondary brake shoe) disposed opposite each other.
An upstream end of the primary brake shoe, which is located upstream when viewed in the forward direction of the rotation of the brake drum, is coupled to the wheel cylinder, while a downstream end of the primary brake shoe is coupled to the upstream end of the secondary brake shoe through, for example, an adjuster. The downstream end of the secondary brake shoe is in contact with an anchor portion fixedly mounted on a backing plate. The anchor portion receives braking reaction forces acting on the primary and secondary brake shoes.
The wheel cylinder presses the primary brake shoe and the secondary brake shoe against a circumferential inner surface of the brake drum. A braking reaction force acting on the primary brake shoe is applied to the upstream end of the secondary brake shoe to press the secondary brake shoe against the circumferential inner surface of the brake drum. Thus, the primary brake shoe and the secondary brake shoe both serve as leading shoes, and the drum brake produces a braking force at very high gain.
The duo servo drum brake is advantageous over the leading trailing drum brake and the two leading drum brake in the following aspects. The duo servo drum brake is capable of producing a fairly high braking effect, and allows for a reduction in the size of the drum brake. Further, it is easy to incorporate the parking brake into the duo servo drum brake.
However, the duo servo drum brake is sensitive to variations in friction coefficients of linings of the brake shoes and a variation of the contact state of the brake drum and the shoes. Therefore, its braking characteristics are unstable.
The leading trailing drum brake is more stable in its braking characteristics, and assembly of the parking brake into the leading trailing drum brake is easy compared to the duo servo drum brake and the two leading drum brake. However, the leading trailing drum brake produces a smaller braking effect. To increase the braking effect, it is necessary to increase the size of a booster of a master cylinder and the diameter of the brake drum. However, this makes it difficult to reduce the size of the drum brake.
As described above, each type of drum brakes have their advantages and disadvantages, and the brakes are selected in accordance with desired vehicle performance, use purposes, production scale, etc.
Automobiles have improved remarkably in their performance over the years. With these improvements, an increase in the braking effect is a key subject in the technical development in this field.
One approach to the problem of low stability of the braking effect of the duo servo drum brake includes controlling the operation of the wheel cylinder by utilizing the anchor reaction force from one of the brake shoes. A control piston is used to open and close a valve that controls a supply of a hydraulic pressure from the master cylinder. The control piston is displaced by the anchor reaction force. This approach has some problems, however. If the anchor reaction force fluctuates and the frequency of the opening and closing of the valve becomes high, the valve is worn out or damaged. If a difference in the hydraulic pressure between the master cylinder and a pressure chamber of the wheel cylinder is large at the time of the opening and closing of the valve, a hydraulic pressure oscillation grows large, and operation of the wheel cylinder becomes unstable. If a complicated structure of the wheel cylinder is used to control wheel cylinder operation in response to the anchor reaction force, the size of the wheel cylinder is increased. The wheel cylinder size increase leads to an increase of the overall size and weight of the drum brake, and to an increase in the cost of manufacturing the wheel cylinder and the drum brake.
In a structure where the anchor reaction force acting on the cylinder body of the wheel cylinder is used to control the wheel cylinder operation, the wheel cylinder body must be constructed of a rigid and expensive material. This also increases the size of the wheel cylinder and the manufacturing cost. As described above, the drum brake is equipped with the anchor portion that receives the anchor reaction force from the brake shoe at the time of braking.
In a conventional anchor portion, a pin that is in contact with the end of the brake shoe is raised on an anchor block. The anchor block is fastened to the cylinder body of the wheel cylinder or to a backing plate. The pin is usually formed integrally with the anchor block by forging or cutting.
Thus, a part of the anchor portion that comes in contact with the end of the brake shoe is in a form of the pin. This avoids a situation where the end of the brake shoe is pressed against the anchor portion, when undesired forces, e.g., twisting, would act on the anchor portion.
A contact surface of the pin tends to wear out, and the pin is easily damaged or broken by its repetitive, strong contact with the end of the brake shoe. To prevent this, the contact surface of the pin that comes into contact with the brake shoe is smoothed by surface finishing or subjected to heat treatment in order to obtain a satisfactory hardness.
The conventional anchor portion has the construction of the pin and the anchor block as described above. Some problems arise from this construction, and some measures have been taken to solve these problems. The shape of the anchor block hinders precise machining and finishing of the pin. A machine used exclusively for the surface finishing of the pin is required. The heat treatment that applies high frequency radiation to the pin only takes a long time. To avoid this, the entire anchor block and the pin may be subjected to the heat treatment. However, the heat treatment then requires a large heat treating equipment. Thus, any of these measures entail an increase of the cost of manufacturing the drum brake.
In the case of the duo servo drum brake, two pins are required. Therefore, when the construction of the pin and the anchor block is employed, the result is an increase in size and complexity of the anchor portion, and the problems referred to above are even more conspicuous.